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From The Collaborative International Dictionary of English v.0.48:
Umbo \Um"bo\, n.; pl. L. Umbones, E. Umbos. [L.] 1. The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike. [1913 Webster] 2. A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane; as, the umbo in the integument of the larvae of echinoderms or in the tympanic membrane of the ear. [1913 Webster] 3. (Zool.) One of the lateral prominences just above the hinge of a bivalve shell. [1913 Webster] Umbonate